You could argue that the ending of ROTJ is more of an emotional book-end to the trilogy, giving off a sense of victory and conclusion, while lore-wise and logically it’s never actually implied that they won the entire war. Movies aren’t about politics, they’re about people and their personal conflicts, and all of these conflicts (e.g. the important one to most viewers) are solved by the end of ROTJ. Luke redeems Vader, Luke finishes his “spiritual” journey, Leia ends up with Han, etc. etc. All we actually see is roughly 20 Rebel pilots celebrating with a bunch of Ewoks. Ok, sure, the SE has the galaxy celebrate victory and freedom, but I don’t think I have to sell the argument that that’s a retcon to anyone here.
However, if you stop and think about what’s going on in the lore, as most people won’t bother to do, then you realize that the end of ROTJ may have been a huge victory, but the galaxy is still filled with the Imperial army and the Rebel Alliance is still a pretty small military force. What ROTJ’s ending sets up lore-wise could easily evolve into the two scenarios we now have; the TFA/Disney one and the old EU one.